5.1 Introduction to Energy and Heat

In my experience, the subject of energy tends to be badly taught in lower-level physics classes. I’ve met a lot of students who, when asked what energy is, can promptly recite the following answer: “Energy is the ability of a system to do work.” Continuing with this website’s theme of trying it yourself, I recommend that you take a field trip to the nearest university campus or high school science class and perform this very experiment, and watch the pride in the students’ eyes as they prove their intelligence to you.

But if you want to throw a bit of a wrench into things, try following up your first question with another: what is work? Many students will frown slightly. Some will say that it is a force exerted over a distance. Others will simply shrug and point at the formula. In other words, most physics students don’t really understand what energy is.

Now here comes the fun part. Go find an actual physicist – a professor, for example – and ask them what energy is. If they give you the same answer about the ability to do work, ask them again, but more emphatically. Say, “Yes, but what really is energy?”

With any luck, you’ll get to watch the professor roll her eyes and glance at a nearby pile of paperwork that is probably more important than stupid questions. Then she’ll look back at you and say, “Well… It’s complicated.”

You see, the most likely reason that the average student doesn’t understand energy is that nobody really understands it – not even the people who are teaching it to others. And I don’t mean that your teachers weren’t paying attention back when they were in school; I mean that the very idea of energy is a completely abstract one, and it is full of mystery even to the most advanced physicists.

The Nobel prize-winning physicist and renowned physics explainer Richard Feynman had the following to say on the topic:

There is a fact, or if you wish, a law, governing all natural phenomena that are known to date. There is no known exception to this law—it is exact so far as we know. The law is called the conservation of energy. It states that there is a certain quantity, which we call energy, that does not change in the manifold changes which nature undergoes. That is a most abstract idea, because it is a mathematical principle; it says that there is a numerical quantity which does not change when something happens. It is not a description of a mechanism, or anything concrete; it is just a strange fact that we can calculate some number and when we finish watching nature go through her tricks and calculate the number again, it is the same. (The Feynman Lectures)

You can almost hear him pleading with us to stop asking questions and to pick up our calculators. But of course, he’s right: energy isn’t anything in particular. It’s a useful mathematical tool, but we don’t really know why. And throughout history, energy has only become more and more mysterious: Einstein showed that what we thought was solid matter could actually turn into pure energy (and vice versa), and in studying heat we eventually discovered the bizarre laws of quantum physics.

In this chapter, we’ll start to learn about this strange abstraction and what it tells us about how things work.